Growth and polarity control of GaN and AlN on carbon-face SiC (C-SiC) by metalorganic vapor phase epitaxy (MOVPE) are reported. The polarities of GaN and AlN layers were found to be strongly dependent on the pre-growth treatment of C-SiC substrates. A pre-flow of trimethyaluminum (TMAl) or a very low NH3/TMAl ratio results in Al(Ga)-polarity layers on C-SiC. Otherwise, N-polarity layers resulted. The polarities of AlN and GaN layers were conveniently determined by their etching rate in KOH or H3PO4, a method reported earlier. We suggest that the Al adatoms, which have a high sticking coefficient on SiC, form several Al adlayers on C-SiC and change the incorporation sequence of Ga(Al) and N leading to metal polarity surface. In addition, the hexagonal pyramids, typical on N-polarity GaN surface, are absent on N-polarity GaN on off-axis C-SiC owing to high density of terraces on off-axis C-SiC. The properties of GaN layers grown on C-SiC are studied by X-ray diffraction.